The art of encoding transmissions so that the transmissions may be authenticated at a receiving module must meet criteria for technical viability (security) as well as low cost and convenience. The cost and convenience criteria result in an inability to use any encoding with polynomials of excessive degree (such as binary numbers of hundreds of bits). Furthermore, cryptographic processing must require less than one second for acceptability by the user. Cost and weight constraints can limit the size and sophistication of a microprocessor or other signal processing equipment used in the system.
An example of such a system is disclosed in commonly owned U.S. Pat. No. 5,191,610 to Hill and Finn. That system utilizes linear feedback shift register pseudorandom number generation having the same seed number and the same, fixed feedback mask in the receiver as in the transmitter. The number of iterations of linear feedback shift register pseudorandom number generation are counted in both the receiver and the transmitter, there being one additional iteration each time that a command is sent. Should the receiver not recognize one of the transmissions (because the transmitter was inadvertently activated at a great distance from the receiver, or otherwise), the receiver is allowed a moderate number of catch-up iterations in which it attempts to match the received transmission. Should that fail, the transmitter tells the receiver how many iterations from the seed it should perform in order to recreate a new current pseudorandom number in order to resynchronize the receiver to the transmitter pseudorandom number.
The aforementioned system requires that a receiver and a transmitter be wired or loaded with a binary feedback mask at the factory and sold as a pair. It also precludes matching a replacement transmitter with an existing receiver without the involvement of dealership personnel, which could compromise security. The pseudorandom number generators of the Hill and Finn patent use one iteration per encrypted message. This saves time but results in a certain level of correlation between successive samples, so that the samples are less random-like. In other pseudorandom number applications, the speed advantage of the aforementioned system could be useful but for the inherent correlation.
Any such system, except one that uses a truly random number of infinite degree, can be compromised either by analysis of a succession of intercepted signals, or by a brute force, exhaustive numerical trial approach which simply tries every number possible as the authentication word (the code or key).
Coded keypads used for unlocking vehicles have inherent security features. The generation of the code word by pressing keys can be shielded from view, and is certainly not capable of being determined beyond a line of sight. Furthermore, there would be great risk for an intruder entering every possible number into a keypad in an attempt to replicate the code (unless, of course, the automobile were parked in an unobservable area, such as a private or otherwise vacant garage). Thus, the keypad cannot be breached by analysis, and is not likely to be breached by numerical trial.
In contrast, lock systems which employ remote transmissions are enormously subject to security tampering because the surveillance of the transmissions may be carried out in another vehicle, without attracting any attention whatsoever. Therefore, it is possible to record many transmissions to a given vehicle, such as in a reserved workplace parking space (which commonly contains expensive cars), as well as providing an unobservable opportunity to attempt the breach of a security system (or even several systems at one time) by broadcasting huge volumes of random numbers, in parking lots where vehicles remain for long periods of time, such as at airports.
Whenever a transmitter is newly assigned to be used with an existing receiver, it is not sufficient to allow the new fob to identify itself and become authorized, without limiting that activity to a time when there is authorized access to the receiver through other than the transmitter itself (that is, within the vehicle itself). Thus, access to the vehicle by means of a traditional key or the like assures the safety of matching a newly assigned transmitter to an existing receiver. In the case of loss of synchronization between the transmitter and the receiver, simply allowing the receiver to synchronize to a particular pseudorandom number provided thereto by the transmitter makes it too easy for a surreptitious breach of security based on the analysis of a few transmissions, and synchronizing thereafter to one of the previous transmissions, utilizing numbers expected to be successful based upon analysis. Mere obfuscation of the resynchronizing code could be compromised by analysis of successful resynchronizations, and determination of the obfuscation function. The danger is not just that a single car might be broken into, but that a sophisticated capability might be developed and thereafter utilized extensively to breach the security of a large number of automobiles of a similar type.